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Book Chapter

Evolution of an Intra-Slope Apron, Offshore Niger Delta Slope: Impact of Step Geometry on Apron Architecture

By
Mark D. Barton
Mark D. Barton
Shell International E and P, 3737 Bellaire Blvd., Houston, Texas 77025, U.S.A.
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Published:
January 01, 2012
Application of the Principles of Seismic Geomorphology to Continental-Slope and Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues. SEPM Special Publication No. 99, Copyright © 2012. SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-304-3, p. 181–197.
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Abstract

A high-resolution 3-D seismic dataset from the offshore Niger delta slope was utilized to study the stratigraphic architecture and evolution of a near-seafloor intraslope apron that overlies an abrupt break in slope. Elements that constitute the apron are from oldest to youngest: (1) a package of prograding lobes, (2) a complex of laterally offset stacked channels, and (3) a sinuous deeply incised bypass channel. Apron evolution reflects the adjustment and response of sediment gravity flows to an evolving slope gradient. Lobes are deposited as flows enter the basin and encounter an abrupt decrease in slope, decelerate, and lose confinement. As the step is healed, flows remain confined and form channels. Eventually, the apron becomes a site of erosion and bypass as down-dip basins become linked by a common graded profile. A comparison with published examples of slope aprons suggests that the geometry of the step may impact the architecture of the apron. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected then aprons formed above severe breaks in slope.

Introduction

Intraslope basins are sites where sand-prone deposits accumulate; they represent a class of economically important hydrocarbon plays around the world (Prather, 2003). Recent sea-floor and near-sea-floor studies of submarine slope fans and aprons have improved our understanding of their gross morphology, evolution, and controls on deposition (Beaubouef and Friedmann, 2000; Fonnesu, 2003; Prather and Pirmez, 2003; Adeogba et al. 2003). However, the internal architecture and depositional elements that comprise stepped slope aprons is not well documented. The objective of this study is to use a high-resolution three-dimensional (3-D) seismic data set to characterize the internal architecture of a near-sea-floor apron that occupies a shallow, stepped basin located along the western Niger delta slope. Deepwater outcrop analogues from similar depositional settings are integrated with the seismic-based interpretations to provide a picture of the geometry, connectivity, and facies architecture of these deposits that is below the resolution of the seismic. This information can be used to better characterize the internal architecture of less well-imaged reservoirs that may have formed in comparable depositional settings.

The Niger Delta, located along the western margin of Africa, forms a symmetrical protrusion into the Gulf of Guinea that covers area of about 210,000 km2 and reaches a maximum thickness of about 12 km (Damuth, 1994). It consists of a regressive sequence of Tertiary clastics that prograded over a passive-continental-margin sequence of mainly Cretaceous sediments. (Doust and Omatsaola, 1990). The submarine slope starts 50–80 km offshore of the coastline at a water depth of about 200 m. Sediment delivered to the submarine slope are sourced from the delta as well as embayments that flank the margins of the delta (Burke, 1972).

Due to rapid sedimentation, the Niger Delta is the site of active loading, growth faulting, and shale remobilization at depth (Allen, 1965; Damuth, 1994; Pirmez et al., 2000; Steffens et al., 2003). As a result, the slope consists of alternating bathymetric highs and lows that (1) affect the path of sediment gravity flows passing through the slope, and (2) act as sediment traps (Prather 2000, 2003). Prather (2003) further classified the Niger Delta slope as an “above-grade, slope that exhibits subtle changes in depositional gradient resulting in low-relief stepped or terraced topography”. The study focused on a shallow intraslope basin located 250 km northwest of the present Niger Delta (Fig. 1). The area covers a 25 km by 15 km region (375 km2) positioned about 80 km downslope from the shelf break in a mid- to lower-slope setting. Water depth increases from approximately 2200 m in the east to over 2800 m in the west.

Fig. 1. –

Map of the Niger Delta region, West Africa. The study area is located 250 km to the northwest of the Niger Delta.

Fig. 1. –

Map of the Niger Delta region, West Africa. The study area is located 250 km to the northwest of the Niger Delta.

Data and Methods

The interval studied was within the upper 350 ms of strata and extended from the sea floor down to a base horizon that extended across the study area. Additional horizons subdivide the interval into major packages. Due to the complexity of the events, each horizon was mapped along every in-line (25 m spacing) for the length of the survey. Amplitude and isochore maps were extracted for each horizon and the intervals between them. Though there are no well data to calibrate the seismic response, high amplitudes in the image (red/yellow) are interpreted to result from high-impedance material, i.e., sands, and zero-crossings in the runsum volumes are interpreted to correspond roughly to contacts between sandstone and mudstone units. The seismic volume has a frequency near 65 Hz and an inline and cross-line spacing of 25 m ×37.5 m. Estimated vertical resolution is about 8 m. Outcrop analogues presented include following: (1) Colleen Canyon, Brushy Canyon Formation, west Texas; (2) Willow Mountain, Bell Canyon Formation, West Texas; (3) Popo Channel, Brushy Canyon Formation, west Texas, (4) Plane Crash Canyon, Brushy Canyon Formation, west Texas, and (5) the Condor West Channel, Cerro Toro Formation, southern Chile.

Slope Morphology

The base horizon is the reflector that defines the base of the system studied. It represents a relatively continuous event on which subsequent events onlap or downlap. The horizon displays a complex physiography composed of high-relief, discontinuous, mud-cored ridges that trend parallel to the dip in slope and low-relief steps that develop orthogonal to the ridges (Fig. 2). Amplitude maps above and below the horizon indicate that the ridges form corridors that funnel sediment into distinct channel fairways that shift location through time (Fig. 3). Below the base horizon, corridor three is occupied by a series of sinuous channel systems while corridors one, two, and four appear inactive. Above the base horizon, corridors one, two, and four are occupied by sinuous channel systems whereas corridor three appears to be inactive. In corridor four a fan-like apron deposit is present beneath the channel systems. This deposit was the primary focus of this study (Fig. 4). An abrupt break in slope separates corridor four into a pair of steps (referred to as steps one and two), defined as the relatively-flat lying portions of the slope, separated by a shallow ramp, defined as the relatively steep portion of the slope (Fig. 5). The lowermost step (step two) covers an area that is about 18 km in length (in a down-dip direction) and 12 km in width (in a direction parallel to the regional slope). It displays an average gradient of around 1.0°. The corresponding ramp displays an average gradient of about 4.0° and a lateral extent of 3–4 km. Steps one and two are linked by a narrow incised channel that is around 400 m in width and up to 30 ms in depth. The fill above the base horizon consists of two main elements: (1) an older, relatively thin (less the 90 ms), funnel-shaped sediment wedge, referred to as a slope apron, and (2) a younger, thicker interval (up to 350 ms) composed of a divergent network of channel–levee complexes.

Fig. 2. –

Base horizon structure map (ms below sea level) of the study area. Depth varies from 2700 ms (red) to 3600 ms (blue) and increases from northeast to the southwest. Horizon displays a complex physiography composed of high-relief ridges that trend parallel to the slope and low-relief steps that develop orthogonal to the mud-cored ridges. Ridges form sediment corridors or fairways that are numbered 1–4.

Fig. 2. –

Base horizon structure map (ms below sea level) of the study area. Depth varies from 2700 ms (red) to 3600 ms (blue) and increases from northeast to the southwest. Horizon displays a complex physiography composed of high-relief ridges that trend parallel to the slope and low-relief steps that develop orthogonal to the mud-cored ridges. Ridges form sediment corridors or fairways that are numbered 1–4.

Fig. 3. –

A) Amplitude map 100 ms below base horizon. Corridor three is occupied by a sinuous channel system that partially spills over into corridor two. Other corridors lack bright amplitudes and appear inactive at this time. B) Amplitude map 100 ms above base horizon. Corridors one, two, and four are occupied by sinuous channel systems; corridor three appears to be inactive.

Fig. 3. –

A) Amplitude map 100 ms below base horizon. Corridor three is occupied by a sinuous channel system that partially spills over into corridor two. Other corridors lack bright amplitudes and appear inactive at this time. B) Amplitude map 100 ms above base horizon. Corridors one, two, and four are occupied by sinuous channel systems; corridor three appears to be inactive.

Fig. 4. –

Amplitude extraction from the base apron horizon. A fanlike apron is present in corridor four beneath the distributive channel system shown in Figure 3B.

Fig. 4. –

Amplitude extraction from the base apron horizon. A fanlike apron is present in corridor four beneath the distributive channel system shown in Figure 3B.

Fig. 5. –

Base horizon map (ms below sea level) from corridor four in the surrounding the area occupied by the fanlike apron shown in Figure 4. The corridor consists of a pair of broad steps separated by a relatively narrow ramp. Step 2 ranges in depth (below sea level) from 3300 ms in the east (boundary marked by red dashed line) to 3600 ms in the west. It is confined on three sides by anticlinal structures to the south and east and by a channel–levee system to the north. Step 2 is linked to step 1 by a narrow incised channel, 400 m wide, that flows from east to west.

Fig. 5. –

Base horizon map (ms below sea level) from corridor four in the surrounding the area occupied by the fanlike apron shown in Figure 4. The corridor consists of a pair of broad steps separated by a relatively narrow ramp. Step 2 ranges in depth (below sea level) from 3300 ms in the east (boundary marked by red dashed line) to 3600 ms in the west. It is confined on three sides by anticlinal structures to the south and east and by a channel–levee system to the north. Step 2 is linked to step 1 by a narrow incised channel, 400 m wide, that flows from east to west.

Apron Architecture and Evolution

The slope apron originates from the incised-channel system located along the ramp between steps one and two narrow (Fig. 6). It is about 10 km in width and 16 km in length. Maximum thickness of the apron is about 90 ms and occurs near the entry point at the transition from ramp to step. The apron spreads out and thins down dip. Internally, it is composed of thicks and thins that display a divergent pattern. In addition, portions of the apron, including the primary exit point for flows leaving step 2, have been erosionally replaced by younger channel–levee systems. The northern portion of the apron is significantly thicker (by about 20 ms) then the southern portion. In cross section the apron displays a wedge geometry that overall thins down dip (Fig. 7). Large portions of the apron have been completely removed by erosion from overlying channel–levee systems. Reflector amplitude and continuity are variable but tend to increase in a down-dip direction within the apron. In strike view, reflectors in the apron converge to the south and on lap to the north (Fig. 8). Their amplitude and continuity appear greater in the southern half of the apron, whereas the base horizon appears more irregular and erosional in the northern half of the apron. Minimum-amplitude maps for the base and top apron horizon are shown in Figure 9. The dimmest amplitudes occur near the sediment entry point and down the axis of the system, whereas the brightest amplitudes are distributed along the flanks of the system.

Fig. 6. –

Apron isochron map. The point of maximum thickness is about 90 ms (red is thick, blue is thin) and occurs at the proximal portion of the apron near the break in slope. The apron is composed of a series of thicks (red) and thins (blues) that diverge and thin basinward. Areas of zero thickness are shaded purple and result from erosion of overlying channel–levee systems.

Fig. 6. –

Apron isochron map. The point of maximum thickness is about 90 ms (red is thick, blue is thin) and occurs at the proximal portion of the apron near the break in slope. The apron is composed of a series of thicks (red) and thins (blues) that diverge and thin basinward. Areas of zero thickness are shaded purple and result from erosion of overlying channel–levee systems.

Fig. 7. –

A) East-to-west dip cross section across apron, illustrating apron geometry and regional slope. The section is about 20 km in length. Location is shown in Figure 6 (see section A–B). B) Apron deposit, shaded purple. The ramp between step 1 and 2 is the result of an anticlinal structure and displays a gradient of about 100 ms per km. The base apron horizon across step 2 displays a gradient of about 15 ms per km, and the top apron horizon displays a gradient of about 20 ms per km.

Fig. 7. –

A) East-to-west dip cross section across apron, illustrating apron geometry and regional slope. The section is about 20 km in length. Location is shown in Figure 6 (see section A–B). B) Apron deposit, shaded purple. The ramp between step 1 and 2 is the result of an anticlinal structure and displays a gradient of about 100 ms per km. The base apron horizon across step 2 displays a gradient of about 15 ms per km, and the top apron horizon displays a gradient of about 20 ms per km.

Fig. 8. –

A) North-to-south strike section across the fan apron. Cross section is about 12 km in length. Location is shown in Figure 6 (see section X–Y’). B) The apron shaded green (lobe-dominated), magenta (bypass channel), and yellow (channel-dominated) is about 10 km in width. It is confined to the south by an anticlinal structure. The northern edge is bounded by a channel–levee system that trends from east to west.

Fig. 8. –

A) North-to-south strike section across the fan apron. Cross section is about 12 km in length. Location is shown in Figure 6 (see section X–Y’). B) The apron shaded green (lobe-dominated), magenta (bypass channel), and yellow (channel-dominated) is about 10 km in width. It is confined to the south by an anticlinal structure. The northern edge is bounded by a channel–levee system that trends from east to west.

Fig. 9. –

Minimum-amplitude map extracted from the base apron horizon. Regions of the apron completely eroded by channel–levee systems indicated by white fill. The brightest amplitudes are interpreted to correspond to relatively sand-rich areas, and the darkest amplitudes (blue and green) to relatively sand-poor areas. Note sinuous amplitude dim that extends down central portion of the apron.

Fig. 9. –

Minimum-amplitude map extracted from the base apron horizon. Regions of the apron completely eroded by channel–levee systems indicated by white fill. The brightest amplitudes are interpreted to correspond to relatively sand-rich areas, and the darkest amplitudes (blue and green) to relatively sand-poor areas. Note sinuous amplitude dim that extends down central portion of the apron.

Based on reflector patterns, amplitudes, and bounding surfaces the apron is subdivided into three distinct packages. Each package represents a discrete phase of deposition, which from oldest to youngest are referred to as (1) a lobe-dominated package, (2) a channel-dominated package, and (3) a bypass-channel package (Fig. 10). The lobe-dominated package dominates the southern and eastern portions of the apron and consists of moderate- to high-amplitude reflectors that display moderate to high continuity. Its basal surface appears conformable to slightly erosional. The channel-dominated package is restricted to the northern part of the apron. The basal surface appears erosional. Reflector amplitude is more variable and less continuous than in the lobe-dominated package. The bypass-channel system is incised into the channel-dominated package to the north and the lobe-dominated package to the south. Reflectors are relatively dim in comparison to the other packages. By volume, the lobe-dominated package makes up about 60 percent of the apron, the channel-dominated package 25 percent, and the bypass channel 15 percent. In a strike sense the packages are laterally offset, with little overlap existing between adjacent packages.

Fig. 10. –

Map of primary stratigraphic elements constituting the apron. The apron consists of three laterally stacked packages listed, in ascending stratigraphic order, a lobe-dominated package (green), a channel-dominated package (yellow), and a bypass-channel system (magenta). Amplitudes display bright divergent patterns in the lobe-dominated package, bright arcuate patterns then the channel-dominated package, and sinuous dims in the bypass-channel systems. Areas where the base horizon has been erosionally modified by overlying channel–levee systems are shaded gray. Areas where the apron is thin (less then 10 ms), and mud-rich (based on extremely dim amplitudes) are shaded white.

Fig. 10. –

Map of primary stratigraphic elements constituting the apron. The apron consists of three laterally stacked packages listed, in ascending stratigraphic order, a lobe-dominated package (green), a channel-dominated package (yellow), and a bypass-channel system (magenta). Amplitudes display bright divergent patterns in the lobe-dominated package, bright arcuate patterns then the channel-dominated package, and sinuous dims in the bypass-channel systems. Areas where the base horizon has been erosionally modified by overlying channel–levee systems are shaded gray. Areas where the apron is thin (less then 10 ms), and mud-rich (based on extremely dim amplitudes) are shaded white.

Lobe-Dominated Package

The lobe-dominated package shows a progressive thinning and spreading from east to west. Reflectors display divergent ribbonlike patterns, interpreted as distributary channels, that pass down dip into broad, fan-shaped amplitudes, interpreted as lobes (Fig. 11). There are ten individual lobe elements. Individual lobe elements are 1–4 km in width and 2–6 km in length. The elements prograde or step basinward, with younger lobe elements often appearing to incise into older lobe elements, especially near the proximal portion of the apron. Each lobe element is associated with a distributive channel system that branches off from a larger, long-reach channel interpreted as a feeder channel (Fig. 11). In cross section the channels display a cuplike geometry near the up-dip portion of the apron and a lens-shaped geometry near the down-dip portion (Fig. 11). The short-reach, radially arranged channels are around 100–200 m in width. The larger, long-reach channels are 400–500 m in width. Channels often display dimmer amplitudes then associated lobe elements, suggesting that they may not be as sand-rich. The larger, long-reach channels are occasionally flanked by broad, apron-shaped elements interpreted as a lateral (Figs. 12, 13). Individual splays range from 1 to 2 km in width and from 2 to 3 km in length.

Fig. 11. –

The lobe-dominated package is composed of 10 lobe elements (green fill with boundaries indicated by dark lines) and a corresponding set of branching, distributary-like, channels (yellow fill). Channels are easily traceable in cross section and display cuplike to lenslike geometries. The main feeder channel (FC) and various branching distributary channels (BDC) are indicated on the cross-sectional views with arrows.

Fig. 11. –

The lobe-dominated package is composed of 10 lobe elements (green fill with boundaries indicated by dark lines) and a corresponding set of branching, distributary-like, channels (yellow fill). Channels are easily traceable in cross section and display cuplike to lenslike geometries. The main feeder channel (FC) and various branching distributary channels (BDC) are indicated on the cross-sectional views with arrows.

Fig. 12. –

A) Minimum amplitude within upper portion of lobe-dominated package adjacent to feeder channel. B) Seismic cross section, location shown in Part A.

Fig. 12. –

A) Minimum amplitude within upper portion of lobe-dominated package adjacent to feeder channel. B) Seismic cross section, location shown in Part A.

Fig. 13. –

A) Event is interpreted as a splay lateral to the main feeder channel. Feeder channel is shaded brown, flanking crevasse splay and channel is shaded red, and underlying lobe channel is shaded yellow. Crevasse channel extends from margin of feeder channel and dissects splay. B) Seismic cross section interpreted.

Fig. 13. –

A) Event is interpreted as a splay lateral to the main feeder channel. Feeder channel is shaded brown, flanking crevasse splay and channel is shaded red, and underlying lobe channel is shaded yellow. Crevasse channel extends from margin of feeder channel and dissects splay. B) Seismic cross section interpreted.

Channel-Dominated Package

The channel-dominated package displays moderate- to high-amplitude reflectors with low continuity (Fig. 14). The package is restricted to the northern half of the apron and displays a width of 2 to 3 km and a thickness that varies from 60 to 80 ms. Within the package, reflector terminations map out as ribbonlike elements that overall trend from west to east (Fig. 15). Several of the elements display arcuate geometries. The elements are interpreted as crosscutting channel elements that form a complex of highly amalgamated channels. The reflector terminations result from the onlap of channel fills onto channel margins, and the truncation of older channel by a younger channel. Individual channel elements range in width from 500 to 750 m. The brightest amplitudes occur near the base of the channels and may represent coarse-grained lag deposits. Dim amplitudes occur near the edges of the channels and may represent fine-grained channel-margin or overbank deposits.

Fig. 14. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section, location shown in Part A.

Fig. 14. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section, location shown in Part A.

Fig. 15. –

A) The channel-dominated package is up to 60 ms in thickness and several kilometers in width. Reflectors display numerous crosscutting relationships interpreted as channels (black dashed lines define channel boundaries). Channels display low-sinuosity, arcuate, map patterns that vary in width from 400 to 750 m. B) Interpreted seismic cross section. To the left of the cross section, the channel complex is eroded and replaced by the bypass channel (purple fill). To the right of the section, the channel complex is eroded and replaced by a younger channel–levee complex (brown fill).

Fig. 15. –

A) The channel-dominated package is up to 60 ms in thickness and several kilometers in width. Reflectors display numerous crosscutting relationships interpreted as channels (black dashed lines define channel boundaries). Channels display low-sinuosity, arcuate, map patterns that vary in width from 400 to 750 m. B) Interpreted seismic cross section. To the left of the cross section, the channel complex is eroded and replaced by the bypass channel (purple fill). To the right of the section, the channel complex is eroded and replaced by a younger channel–levee complex (brown fill).

Bypass-Channel Package

The bypass channel is a deeply incised, throughgoing channel system that dissects the apron into a northern channel-dominated package and a southern lobe-dominated package (Figs. 16, 17). The package is up to 2 km in width and 90 ms in depth. It consists of three parts: (1) a narrow, sinuous, low-amplitude, throughgoing channel element, (2) a series of linear- to pod-shaped high-amplitude reflectors located at the inside bends of the sinuous channel, and (3) a region of chaotic, low-amplitude reflectors that flank the margins of the sinuous channel. The sinuous channel element is 300 to 400 m in width. Depth increases in a down-dip direction from about 40 ms at the entry point to about 80 ms at the exit point. About two kilometers down dip from the entry point the depth of the channel abruptly increases from about 45 to 65 ms. The abrupt increase in incision is interpreted as an up-dip-migrating knickpoint. The knickpoint represents a drop in base level and readjustment of the equilibrium profile through erosion and the base of the channel (Adeogba, 2003). Up dip of the knickpoint, the channel is relatively straight, while down dip of the knickpoint it becomes progressively more sinuous. The pattern suggests that the up-dip portion of the channel was deepening by way of knickpoint migration and erosion, while the down-dip portion of the channel had achieved base level and was widening by lateral channel migration. The high-amplitude pods, present at the base of the bypass-channel package, are deposits of the laterally migrating channel. In cross section, high-angle truncations can be mapped that follow the direction of channel migration (Fig. 17B). The deposits are believed to consist of very coarse-grained material, reworked from previous sediments, by flows that for the most part passed though the system. Chaotic, low-amplitude reflectors that flank the channel are interpreted as fine-grained slump and inner-levee deposits. The depth of incision and the presence of a throughgoing channel is evidence of sediment bypass to the next sub-basin lower on the slope (Adeogba, 2003).

Fig. 16. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section; location shown in Part A.

Fig. 16. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section; location shown in Part A.

Fig. 17. –

A) The bypass channel is characterized by sinuous amplitude dim that crosscuts the entire apron. The bypass-channel complex is up to 2000 m in width and 70 ms in thickness. The final channel fill is 250 to 350 m in width. The channel is deeper and more sinuous down-dip of an apparent knickpoint (see arrow). The knickpoint is suggested by an abrupt downstream (to the west) increase in channel depth of 25 to 30 ms. Dim amplitudes indicate that much of the channel is filled with relatively mud-rich deposits. Amplitude patterns down dip of the knickpoint shows a meandering channel pattern with the development of point-bar-like deposits. B) Interpreted seismic cross section. The bypass channel (shaded pink and gray fill) deeply incises through the previous apron deposits (shaded white). Wedge-shaped, low-amplitude reflectors flank the channel and are interpreted as levees (brown fill). High-angle truncations (indicated by blue dashed lines) indicate that channel migration was from left to right relative to the figure and punctuated by several episodes of cut and fill (see strike cross section, increments 20 ms).

Fig. 17. –

A) The bypass channel is characterized by sinuous amplitude dim that crosscuts the entire apron. The bypass-channel complex is up to 2000 m in width and 70 ms in thickness. The final channel fill is 250 to 350 m in width. The channel is deeper and more sinuous down-dip of an apparent knickpoint (see arrow). The knickpoint is suggested by an abrupt downstream (to the west) increase in channel depth of 25 to 30 ms. Dim amplitudes indicate that much of the channel is filled with relatively mud-rich deposits. Amplitude patterns down dip of the knickpoint shows a meandering channel pattern with the development of point-bar-like deposits. B) Interpreted seismic cross section. The bypass channel (shaded pink and gray fill) deeply incises through the previous apron deposits (shaded white). Wedge-shaped, low-amplitude reflectors flank the channel and are interpreted as levees (brown fill). High-angle truncations (indicated by blue dashed lines) indicate that channel migration was from left to right relative to the figure and punctuated by several episodes of cut and fill (see strike cross section, increments 20 ms).

Channel–Levee Architecture and Evolution

The interval between the sea floor and the top apron horizon consists of a distributive network of channel–levee complexes (Figs. 18, 19). The channel–levee complexes are up to 300 ms in thickness (measured from base of channel to levee crest) and 2– 4 km in width (measured between levee crests). Individual channel elements are 200–500 m in width and highly sinuous. Cross-cutting relationships indicate that the channel–levee complexes are not contemporaneous but rather separate events related to abrupt lateral shifts in the position of the channel through time due to avulsion. Three main channel–levee complexes, numbered 1 through 3 in ascending stratigraphic order, are identified. The position of the channels appears to be controlled by topography, with channels converging across the ramp and diverging across the step. Avulsion points are located near breaks in slope. The first avulsion point occurs near the western edge of step 1 and results in channel system (CLC-1) being diverted to the north of the apron bypass channel. The second and third avulsion points occur at the ramp-to-step transition above step 2 and result in channel systems CLC-2 and CLC-3 being diverted to the south. The avulsions also appear to occur after a period of channel aggradation. Within channel–levee elements 1 and 2, the elevation of the final active channel fill is about 50 ms below the levee crest and nearly 250 ms above the erosional base of the system (Fig. 15B). Each of the channel systems (CLC-1, CLC-2, and CLC-3) incise through the underlying apron. HARP-like deposits, originally defined as high-amplitude reflection packets (Pirmez et. al., 1997), are associated with channel–levee complex 3. The deposits display funnel-shaped geometry, up to 100 ms in thickness and 8 km in width. They are confined by an anticlinal structure to the south and by topography created by channel– levee complex 2 to the north. The internal architecture of the deposits was not investigated, but the base looks erosional and the internal character appears channelized.

Fig. 18. –

A) Minimum-amplitude map of the interval between the sea floor and the top apron horizon. B) North-to-south cross section across the study area. B) Seismic cross section; location shown in Part A.

Fig. 18. –

A) Minimum-amplitude map of the interval between the sea floor and the top apron horizon. B) North-to-south cross section across the study area. B) Seismic cross section; location shown in Part A.

Fig. 19. –

A) Interpretation of interval shown in Figure 18A, showing three main channel–levee complexes numbered 1 through 3 in ascending stratigraphic order. The channel–levee complexes display a distributive or divergent pattern across the step, with avulsion points occurring near the transition from ramp to step. The channel complex 3 consists of a broad, apron-shaped deposit incised by a sinuous channel system. B) In cross section, the channel–levee systems are numbered and color-coded in the same fashion as on the base map. The underlying apron deposit is colored purple. The channel–levee systems are up to 300 ms in thickness (measured from base of channel to levee crest) and several kilometers in width.

Fig. 19. –

A) Interpretation of interval shown in Figure 18A, showing three main channel–levee complexes numbered 1 through 3 in ascending stratigraphic order. The channel–levee complexes display a distributive or divergent pattern across the step, with avulsion points occurring near the transition from ramp to step. The channel complex 3 consists of a broad, apron-shaped deposit incised by a sinuous channel system. B) In cross section, the channel–levee systems are numbered and color-coded in the same fashion as on the base map. The underlying apron deposit is colored purple. The channel–levee systems are up to 300 ms in thickness (measured from base of channel to levee crest) and several kilometers in width.

Discussion

The slope apron at OPL315 evolved in three distinct phases that correspond to a lobe-dominated package, a channel-dominated package, and bypass-channel system. Each phase is interpreted to reflect the adjustment and response of sediment gravity flows to an evolving slope gradient (Fig. 20). Deposition began after subsidence and the formation of a shallow, stepped basin. A lobe-dominated slope apron is formed as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. As the slope break is healed, and a local graded profile achieved, apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. Over time, down-dip basins become linked by a regional or common graded profile and the step becomes a site of erosion and bypass. Flows consolidate into a single throughgoing bypass channel that shows evidence of significant incision and enlargement. The model is similar to previously described models for slope aprons (Beaubouef and Friedmann, 2000; Prather, 2003; Adeogba, 2003). The primary difference is the development of a local graded profile that results in significant portions of the apron being channelized.

Fig. 20. –

Model describing the evolution of the intraslope apron at OPL 315. A) Local subsidence results in the development of a stepped slope profile. B) A lobe-dominated slope wedge forms as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. C) The slope break is healed, a local graded profile is achieved, and apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. D) Down-dip basins become linked by a regional or common graded profile, and the step becomes a site of incision and bypass.

Fig. 20. –

Model describing the evolution of the intraslope apron at OPL 315. A) Local subsidence results in the development of a stepped slope profile. B) A lobe-dominated slope wedge forms as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. C) The slope break is healed, a local graded profile is achieved, and apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. D) Down-dip basins become linked by a regional or common graded profile, and the step becomes a site of incision and bypass.

A comparison with published examples of slope aprons, such as OPL 211 (Prather et al., 2007) shows significant differences in geometry and architecture (Fig. 21). Differences include: (1) the geometry of the ramp and step, (2) the stacking pattern of depositional units, (3) internal architectures, and (4) the degree of dissection. At OPL 211 the ramp is steeper (80 ms/km vs. 55 ms/km), the step flatter (0 ms/km vs. 10 ms/km), and the apron thicker (120 ms vs. 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel–lobe complexes. At OPL 315, initial depositional units consist of distributive channel–lobe complexes while later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.

Fig. 21. –

Slope aprons from OPL 211 and OPL 315 (this study) are compared in terms of step geometry, stacking pattern of depositional units, and internal architecture. At OPL 211 the ramp is steeper (80 ms/km versus 55 ms/km), the step flatter (0 ms/km versus 10 ms/km), and the apron thicker (120 ms versus 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel– lobe complexes. At OPL 315, initial deposi-tional units consist of distributive channel– lobe complexes, whereas later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.

Fig. 21. –

Slope aprons from OPL 211 and OPL 315 (this study) are compared in terms of step geometry, stacking pattern of depositional units, and internal architecture. At OPL 211 the ramp is steeper (80 ms/km versus 55 ms/km), the step flatter (0 ms/km versus 10 ms/km), and the apron thicker (120 ms versus 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel– lobe complexes. At OPL 315, initial deposi-tional units consist of distributive channel– lobe complexes, whereas later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.

Differences in element geometries and stacking patterns between the two systems may result from variations in the steepness of the slope and corresponding step (Fig. 22). Sediment gravity flows encountering a sharp break in slope, such as OPL 211, which displays a steep slope passing into a flat step, are likely to transfer a larger component of the flow energy laterally than sediment gravity flows encountering a mild or shallow break in slope, such as OPL 315, which displays a shallow slope passing into a slightly dipping step. In the OPL 211 case, the greater lateral transfer of energy at the slope break causes the flow to lose confinement, channels become unstable, and lobes are deposited in an aggradational to laterally stacked fashion across the break in slope. By contrast, in the OPL 315 case, the relatively shallow break in slope allows flow energy to be transferred basinward. As a result, the flow remains confined, channels maintain stability, and lobes are deposited in a progradational fashion across the step.

Fig. 22. –

Diagram illustrating the impact of ramp geometry on flow character. Sediment gravity flows that pass over a steep ramp or a sharp break in slope (such as OPL 211) transfer a larger proportion of the flow energy laterally than flows that pass over a shallow ramp or a mild break in slope. The sudden transfer of energy laterally causes the flow to rapidly lose confinement and deposit lobes. Lobes continue to stack laterally and vertically as the lateral transfer of energy prevents channel formation and progradation across the step. In contrast, flows that encounter a mild break in slope (such as OPL 315) transfer the greater part of the flow energy basinward. Channels remain stable, with flows gradually losing confinement. As a result, lobes are deposited in progradational fashion across the step.

Fig. 22. –

Diagram illustrating the impact of ramp geometry on flow character. Sediment gravity flows that pass over a steep ramp or a sharp break in slope (such as OPL 211) transfer a larger proportion of the flow energy laterally than flows that pass over a shallow ramp or a mild break in slope. The sudden transfer of energy laterally causes the flow to rapidly lose confinement and deposit lobes. Lobes continue to stack laterally and vertically as the lateral transfer of energy prevents channel formation and progradation across the step. In contrast, flows that encounter a mild break in slope (such as OPL 315) transfer the greater part of the flow energy basinward. Channels remain stable, with flows gradually losing confinement. As a result, lobes are deposited in progradational fashion across the step.

Summary

Two types of systems are seen across the step slope in the study area: a slope apron, and a distributive network of channel–levee complexes. Deposition of the slope apron began after subsidence and the formation of a shallow stepped basin. The apron consists of three distinct phases of sedimentation: a lobedominated package, a channel-dominated package, and a bypass channel. A lobe-dominated slope apron is formed as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. As the slope break is healed, and a local graded profile achieved, apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. Over time, down-dip basins become linked by a regional or common graded profile and the step becomes a site of erosion and bypass. Flows consolidate into a single throughgoing bypass channel that shows evidence of significant incision and enlargement. Differences in apron architecture may reflect difference in the geometry of the step. Aprons formed above mild breaks in slopes should be thinner, more channelized, and potentially more dissected than aprons formed above severe breaks in slope.

References

Adeogba
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A.A.
,
2003
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Depositional controls and geometries of deep-water deposits as imaged by near-surface 3-D seismic data, Niger Delta, Nigeria
 : M.S. Dissertation,
Stanford University
,
Stanford, California
, 82 p.
Allen
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Friedmann
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Weimer
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Abreu
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Van Wagoner
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Burke
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Damuth
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Neogene gravity tectonics and depositional processes on the deep Niger Delta Continental Margin
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Fonnesu
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Acknowledgments

I thank Petroleum Geo-Services for allowing seismic data utilized in this study to be published. The paper benefited greatly from discussions with Alesandro Cantelli, Carlos Pirmez, Brad Prather, and Ciaran O’Byrne. Support for this project provided by Shell Exploration and Production, including managers Steve Tennant, Mark Hempton, Steve Naruk, and Marc Alberts.

Figures & Tables

Fig. 1. –

Map of the Niger Delta region, West Africa. The study area is located 250 km to the northwest of the Niger Delta.

Fig. 1. –

Map of the Niger Delta region, West Africa. The study area is located 250 km to the northwest of the Niger Delta.

Fig. 2. –

Base horizon structure map (ms below sea level) of the study area. Depth varies from 2700 ms (red) to 3600 ms (blue) and increases from northeast to the southwest. Horizon displays a complex physiography composed of high-relief ridges that trend parallel to the slope and low-relief steps that develop orthogonal to the mud-cored ridges. Ridges form sediment corridors or fairways that are numbered 1–4.

Fig. 2. –

Base horizon structure map (ms below sea level) of the study area. Depth varies from 2700 ms (red) to 3600 ms (blue) and increases from northeast to the southwest. Horizon displays a complex physiography composed of high-relief ridges that trend parallel to the slope and low-relief steps that develop orthogonal to the mud-cored ridges. Ridges form sediment corridors or fairways that are numbered 1–4.

Fig. 3. –

A) Amplitude map 100 ms below base horizon. Corridor three is occupied by a sinuous channel system that partially spills over into corridor two. Other corridors lack bright amplitudes and appear inactive at this time. B) Amplitude map 100 ms above base horizon. Corridors one, two, and four are occupied by sinuous channel systems; corridor three appears to be inactive.

Fig. 3. –

A) Amplitude map 100 ms below base horizon. Corridor three is occupied by a sinuous channel system that partially spills over into corridor two. Other corridors lack bright amplitudes and appear inactive at this time. B) Amplitude map 100 ms above base horizon. Corridors one, two, and four are occupied by sinuous channel systems; corridor three appears to be inactive.

Fig. 4. –

Amplitude extraction from the base apron horizon. A fanlike apron is present in corridor four beneath the distributive channel system shown in Figure 3B.

Fig. 4. –

Amplitude extraction from the base apron horizon. A fanlike apron is present in corridor four beneath the distributive channel system shown in Figure 3B.

Fig. 5. –

Base horizon map (ms below sea level) from corridor four in the surrounding the area occupied by the fanlike apron shown in Figure 4. The corridor consists of a pair of broad steps separated by a relatively narrow ramp. Step 2 ranges in depth (below sea level) from 3300 ms in the east (boundary marked by red dashed line) to 3600 ms in the west. It is confined on three sides by anticlinal structures to the south and east and by a channel–levee system to the north. Step 2 is linked to step 1 by a narrow incised channel, 400 m wide, that flows from east to west.

Fig. 5. –

Base horizon map (ms below sea level) from corridor four in the surrounding the area occupied by the fanlike apron shown in Figure 4. The corridor consists of a pair of broad steps separated by a relatively narrow ramp. Step 2 ranges in depth (below sea level) from 3300 ms in the east (boundary marked by red dashed line) to 3600 ms in the west. It is confined on three sides by anticlinal structures to the south and east and by a channel–levee system to the north. Step 2 is linked to step 1 by a narrow incised channel, 400 m wide, that flows from east to west.

Fig. 6. –

Apron isochron map. The point of maximum thickness is about 90 ms (red is thick, blue is thin) and occurs at the proximal portion of the apron near the break in slope. The apron is composed of a series of thicks (red) and thins (blues) that diverge and thin basinward. Areas of zero thickness are shaded purple and result from erosion of overlying channel–levee systems.

Fig. 6. –

Apron isochron map. The point of maximum thickness is about 90 ms (red is thick, blue is thin) and occurs at the proximal portion of the apron near the break in slope. The apron is composed of a series of thicks (red) and thins (blues) that diverge and thin basinward. Areas of zero thickness are shaded purple and result from erosion of overlying channel–levee systems.

Fig. 7. –

A) East-to-west dip cross section across apron, illustrating apron geometry and regional slope. The section is about 20 km in length. Location is shown in Figure 6 (see section A–B). B) Apron deposit, shaded purple. The ramp between step 1 and 2 is the result of an anticlinal structure and displays a gradient of about 100 ms per km. The base apron horizon across step 2 displays a gradient of about 15 ms per km, and the top apron horizon displays a gradient of about 20 ms per km.

Fig. 7. –

A) East-to-west dip cross section across apron, illustrating apron geometry and regional slope. The section is about 20 km in length. Location is shown in Figure 6 (see section A–B). B) Apron deposit, shaded purple. The ramp between step 1 and 2 is the result of an anticlinal structure and displays a gradient of about 100 ms per km. The base apron horizon across step 2 displays a gradient of about 15 ms per km, and the top apron horizon displays a gradient of about 20 ms per km.

Fig. 8. –

A) North-to-south strike section across the fan apron. Cross section is about 12 km in length. Location is shown in Figure 6 (see section X–Y’). B) The apron shaded green (lobe-dominated), magenta (bypass channel), and yellow (channel-dominated) is about 10 km in width. It is confined to the south by an anticlinal structure. The northern edge is bounded by a channel–levee system that trends from east to west.

Fig. 8. –

A) North-to-south strike section across the fan apron. Cross section is about 12 km in length. Location is shown in Figure 6 (see section X–Y’). B) The apron shaded green (lobe-dominated), magenta (bypass channel), and yellow (channel-dominated) is about 10 km in width. It is confined to the south by an anticlinal structure. The northern edge is bounded by a channel–levee system that trends from east to west.

Fig. 9. –

Minimum-amplitude map extracted from the base apron horizon. Regions of the apron completely eroded by channel–levee systems indicated by white fill. The brightest amplitudes are interpreted to correspond to relatively sand-rich areas, and the darkest amplitudes (blue and green) to relatively sand-poor areas. Note sinuous amplitude dim that extends down central portion of the apron.

Fig. 9. –

Minimum-amplitude map extracted from the base apron horizon. Regions of the apron completely eroded by channel–levee systems indicated by white fill. The brightest amplitudes are interpreted to correspond to relatively sand-rich areas, and the darkest amplitudes (blue and green) to relatively sand-poor areas. Note sinuous amplitude dim that extends down central portion of the apron.

Fig. 10. –

Map of primary stratigraphic elements constituting the apron. The apron consists of three laterally stacked packages listed, in ascending stratigraphic order, a lobe-dominated package (green), a channel-dominated package (yellow), and a bypass-channel system (magenta). Amplitudes display bright divergent patterns in the lobe-dominated package, bright arcuate patterns then the channel-dominated package, and sinuous dims in the bypass-channel systems. Areas where the base horizon has been erosionally modified by overlying channel–levee systems are shaded gray. Areas where the apron is thin (less then 10 ms), and mud-rich (based on extremely dim amplitudes) are shaded white.

Fig. 10. –

Map of primary stratigraphic elements constituting the apron. The apron consists of three laterally stacked packages listed, in ascending stratigraphic order, a lobe-dominated package (green), a channel-dominated package (yellow), and a bypass-channel system (magenta). Amplitudes display bright divergent patterns in the lobe-dominated package, bright arcuate patterns then the channel-dominated package, and sinuous dims in the bypass-channel systems. Areas where the base horizon has been erosionally modified by overlying channel–levee systems are shaded gray. Areas where the apron is thin (less then 10 ms), and mud-rich (based on extremely dim amplitudes) are shaded white.

Fig. 11. –

The lobe-dominated package is composed of 10 lobe elements (green fill with boundaries indicated by dark lines) and a corresponding set of branching, distributary-like, channels (yellow fill). Channels are easily traceable in cross section and display cuplike to lenslike geometries. The main feeder channel (FC) and various branching distributary channels (BDC) are indicated on the cross-sectional views with arrows.

Fig. 11. –

The lobe-dominated package is composed of 10 lobe elements (green fill with boundaries indicated by dark lines) and a corresponding set of branching, distributary-like, channels (yellow fill). Channels are easily traceable in cross section and display cuplike to lenslike geometries. The main feeder channel (FC) and various branching distributary channels (BDC) are indicated on the cross-sectional views with arrows.

Fig. 12. –

A) Minimum amplitude within upper portion of lobe-dominated package adjacent to feeder channel. B) Seismic cross section, location shown in Part A.

Fig. 12. –

A) Minimum amplitude within upper portion of lobe-dominated package adjacent to feeder channel. B) Seismic cross section, location shown in Part A.

Fig. 13. –

A) Event is interpreted as a splay lateral to the main feeder channel. Feeder channel is shaded brown, flanking crevasse splay and channel is shaded red, and underlying lobe channel is shaded yellow. Crevasse channel extends from margin of feeder channel and dissects splay. B) Seismic cross section interpreted.

Fig. 13. –

A) Event is interpreted as a splay lateral to the main feeder channel. Feeder channel is shaded brown, flanking crevasse splay and channel is shaded red, and underlying lobe channel is shaded yellow. Crevasse channel extends from margin of feeder channel and dissects splay. B) Seismic cross section interpreted.

Fig. 14. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section, location shown in Part A.

Fig. 14. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section, location shown in Part A.

Fig. 15. –

A) The channel-dominated package is up to 60 ms in thickness and several kilometers in width. Reflectors display numerous crosscutting relationships interpreted as channels (black dashed lines define channel boundaries). Channels display low-sinuosity, arcuate, map patterns that vary in width from 400 to 750 m. B) Interpreted seismic cross section. To the left of the cross section, the channel complex is eroded and replaced by the bypass channel (purple fill). To the right of the section, the channel complex is eroded and replaced by a younger channel–levee complex (brown fill).

Fig. 15. –

A) The channel-dominated package is up to 60 ms in thickness and several kilometers in width. Reflectors display numerous crosscutting relationships interpreted as channels (black dashed lines define channel boundaries). Channels display low-sinuosity, arcuate, map patterns that vary in width from 400 to 750 m. B) Interpreted seismic cross section. To the left of the cross section, the channel complex is eroded and replaced by the bypass channel (purple fill). To the right of the section, the channel complex is eroded and replaced by a younger channel–levee complex (brown fill).

Fig. 16. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section; location shown in Part A.

Fig. 16. –

A) Minimum amplitude of channel-dominated package. B) Seismic cross section; location shown in Part A.

Fig. 17. –

A) The bypass channel is characterized by sinuous amplitude dim that crosscuts the entire apron. The bypass-channel complex is up to 2000 m in width and 70 ms in thickness. The final channel fill is 250 to 350 m in width. The channel is deeper and more sinuous down-dip of an apparent knickpoint (see arrow). The knickpoint is suggested by an abrupt downstream (to the west) increase in channel depth of 25 to 30 ms. Dim amplitudes indicate that much of the channel is filled with relatively mud-rich deposits. Amplitude patterns down dip of the knickpoint shows a meandering channel pattern with the development of point-bar-like deposits. B) Interpreted seismic cross section. The bypass channel (shaded pink and gray fill) deeply incises through the previous apron deposits (shaded white). Wedge-shaped, low-amplitude reflectors flank the channel and are interpreted as levees (brown fill). High-angle truncations (indicated by blue dashed lines) indicate that channel migration was from left to right relative to the figure and punctuated by several episodes of cut and fill (see strike cross section, increments 20 ms).

Fig. 17. –

A) The bypass channel is characterized by sinuous amplitude dim that crosscuts the entire apron. The bypass-channel complex is up to 2000 m in width and 70 ms in thickness. The final channel fill is 250 to 350 m in width. The channel is deeper and more sinuous down-dip of an apparent knickpoint (see arrow). The knickpoint is suggested by an abrupt downstream (to the west) increase in channel depth of 25 to 30 ms. Dim amplitudes indicate that much of the channel is filled with relatively mud-rich deposits. Amplitude patterns down dip of the knickpoint shows a meandering channel pattern with the development of point-bar-like deposits. B) Interpreted seismic cross section. The bypass channel (shaded pink and gray fill) deeply incises through the previous apron deposits (shaded white). Wedge-shaped, low-amplitude reflectors flank the channel and are interpreted as levees (brown fill). High-angle truncations (indicated by blue dashed lines) indicate that channel migration was from left to right relative to the figure and punctuated by several episodes of cut and fill (see strike cross section, increments 20 ms).

Fig. 18. –

A) Minimum-amplitude map of the interval between the sea floor and the top apron horizon. B) North-to-south cross section across the study area. B) Seismic cross section; location shown in Part A.

Fig. 18. –

A) Minimum-amplitude map of the interval between the sea floor and the top apron horizon. B) North-to-south cross section across the study area. B) Seismic cross section; location shown in Part A.

Fig. 19. –

A) Interpretation of interval shown in Figure 18A, showing three main channel–levee complexes numbered 1 through 3 in ascending stratigraphic order. The channel–levee complexes display a distributive or divergent pattern across the step, with avulsion points occurring near the transition from ramp to step. The channel complex 3 consists of a broad, apron-shaped deposit incised by a sinuous channel system. B) In cross section, the channel–levee systems are numbered and color-coded in the same fashion as on the base map. The underlying apron deposit is colored purple. The channel–levee systems are up to 300 ms in thickness (measured from base of channel to levee crest) and several kilometers in width.

Fig. 19. –

A) Interpretation of interval shown in Figure 18A, showing three main channel–levee complexes numbered 1 through 3 in ascending stratigraphic order. The channel–levee complexes display a distributive or divergent pattern across the step, with avulsion points occurring near the transition from ramp to step. The channel complex 3 consists of a broad, apron-shaped deposit incised by a sinuous channel system. B) In cross section, the channel–levee systems are numbered and color-coded in the same fashion as on the base map. The underlying apron deposit is colored purple. The channel–levee systems are up to 300 ms in thickness (measured from base of channel to levee crest) and several kilometers in width.

Fig. 20. –

Model describing the evolution of the intraslope apron at OPL 315. A) Local subsidence results in the development of a stepped slope profile. B) A lobe-dominated slope wedge forms as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. C) The slope break is healed, a local graded profile is achieved, and apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. D) Down-dip basins become linked by a regional or common graded profile, and the step becomes a site of incision and bypass.

Fig. 20. –

Model describing the evolution of the intraslope apron at OPL 315. A) Local subsidence results in the development of a stepped slope profile. B) A lobe-dominated slope wedge forms as originally confined sediment gravity flows encounter an abrupt decrease in slope, decelerate, lose confinement, and deposit their load. C) The slope break is healed, a local graded profile is achieved, and apron aggradation ceases. Incoming sediment gravity flows remain confined, eroding and replacing portions of the apron by channels. D) Down-dip basins become linked by a regional or common graded profile, and the step becomes a site of incision and bypass.

Fig. 21. –

Slope aprons from OPL 211 and OPL 315 (this study) are compared in terms of step geometry, stacking pattern of depositional units, and internal architecture. At OPL 211 the ramp is steeper (80 ms/km versus 55 ms/km), the step flatter (0 ms/km versus 10 ms/km), and the apron thicker (120 ms versus 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel– lobe complexes. At OPL 315, initial deposi-tional units consist of distributive channel– lobe complexes, whereas later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.

Fig. 21. –

Slope aprons from OPL 211 and OPL 315 (this study) are compared in terms of step geometry, stacking pattern of depositional units, and internal architecture. At OPL 211 the ramp is steeper (80 ms/km versus 55 ms/km), the step flatter (0 ms/km versus 10 ms/km), and the apron thicker (120 ms versus 80 ms), than at OPL 315. At OPL 211 the apron is made up of vertically stacked, progradational units. At OPL 315, the units erode and stack in a lateral fashion. Internally, depositional units at OPL 211 consist of distributive channel– lobe complexes. At OPL 315, initial deposi-tional units consist of distributive channel– lobe complexes, whereas later units consist of channel complexes. The apron at OPL 315 is completely dissected by the bypass channel, whereas dissection of the OPL 211 apron is focused near the entry and exit points.

Fig. 22. –

Diagram illustrating the impact of ramp geometry on flow character. Sediment gravity flows that pass over a steep ramp or a sharp break in slope (such as OPL 211) transfer a larger proportion of the flow energy laterally than flows that pass over a shallow ramp or a mild break in slope. The sudden transfer of energy laterally causes the flow to rapidly lose confinement and deposit lobes. Lobes continue to stack laterally and vertically as the lateral transfer of energy prevents channel formation and progradation across the step. In contrast, flows that encounter a mild break in slope (such as OPL 315) transfer the greater part of the flow energy basinward. Channels remain stable, with flows gradually losing confinement. As a result, lobes are deposited in progradational fashion across the step.

Fig. 22. –

Diagram illustrating the impact of ramp geometry on flow character. Sediment gravity flows that pass over a steep ramp or a sharp break in slope (such as OPL 211) transfer a larger proportion of the flow energy laterally than flows that pass over a shallow ramp or a mild break in slope. The sudden transfer of energy laterally causes the flow to rapidly lose confinement and deposit lobes. Lobes continue to stack laterally and vertically as the lateral transfer of energy prevents channel formation and progradation across the step. In contrast, flows that encounter a mild break in slope (such as OPL 315) transfer the greater part of the flow energy basinward. Channels remain stable, with flows gradually losing confinement. As a result, lobes are deposited in progradational fashion across the step.

Contents

SEPM Special Publication

Application of the Principles of Seismic Geomorphology to Continental Slope and Base-of-Slope Systems: Case Studies from SeaFloor and Near-Sea Floor Analogues

SEPM Society for Sedimentary Geology
Volume
99
ISBN electronic:
9781565763043
Publication date:
January 01, 2012

GeoRef

References

References

Adeogba
,
A.A.
,
2003
,
Depositional controls and geometries of deep-water deposits as imaged by near-surface 3-D seismic data, Niger Delta, Nigeria
 : M.S. Dissertation,
Stanford University
,
Stanford, California
, 82 p.
Allen
,
J.R.L.
,
1965
,
Late Quaternary Niger Delta, and adjacent areas— sedimentary environments and lithofacies
:
American Association of Petroleum Geologists, Bulletin
 , v.
49
, p.
547
800
Beaubouef
,
R.T.
Friedmann
,
S.J.
,
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